In recent years, the wireless power transmission technology is employed for many devices such as IC cards and mobile telephones. The wireless power transmission technology wirelessly and contactlessly transmits electric power using a power transmission coil and a power receiving coil. However, transmission characteristics greatly vary with positional relation between the power transmission coil and the power receiving coil. In the status quo, for example, a cradle fixes the positional relation between both coils so as to limit the distance between both coils to zero to several centimeters.
It is necessary to detect transmission characteristics depending on coil positions and adjust circuit parameters in order to ensure the transmission distance of several tens of centimeters or longer without limiting the positional relation between both coils. To do this, there is proposed a method that derives an electromagnetic coupling coefficient using the circuit sensor and the table provided for the power transmitter to adjust the tuning capacitor, improves the power transmission efficiency, and reduces heating.
However, the method aims to maximize the receiving power in a limited situation where the electromagnetic coupling coefficient approximates to 0.1. There is no known a method of ensuring high power transmission efficiency even in a wireless power transmission system that allows the electromagnetic coupling coefficient to dynamically vary from approximately 1 as an ideal coupling state to smaller than 0.001 as a very weak coupling state.
It has been difficult for the related art to ensure high power transmission efficiency in a wireless power transmission system where the electromagnetic coupling coefficient varies greatly.